BMW OEM - Clutch Slave Cylinder, OEM - E46 incl M3, X3, Z4, Z4M (21526766279)

This is the OEM clutch slave cylinder for all E46 including the E46 M3 and all Z4, including the Z4 M Roadster/Coupe. A failed clutch slave cylinder will cause the clutch pedal to fall to the floor with no pressure. There's no pressure inside the lines because the seals inside the slave cylinder have failed. Unfortunately, this happens with almost no warning although if there is a spongy feeling or excessive slop in the clutch pedal travel it could be a sign of an impending failure (or simply air in the system).

Replacing the slave cylinder during other maintenance, such as a clutch replacement, is the best way to not leave you stranded.

This is the OEM E46, Z4 clutch slave cylinder, made by FTE or Sachs, an original supplier to BMW for many clutch components. 

This clutch slave is the latest most up-to-date design, interchangeable with BMW part numbers: 
21526785966 (21 52 6 785 966)
21526775922 (21 52 6 775 922)
21526766279 (21 52 6 766 279) 
21521164919 (21 52 1 164 919)